The present disclosure relates to a brushholder for an electric motor, in particular a DC motor, which comprises a carbon cartridge which forms an accommodating area with a lateral surface, wherein a carbon brush is arranged in the accommodating area and has an end face and a side face, wherein the brushholder additionally comprises a spring, which rests on the end face and presses the carbon brush in a direction of extent of the accommodating area. The present disclosure furthermore relates to a handheld machine tool comprising an electric motor which has a brushholder according to the disclosure.
Electric tools are usually driven using universal electric motors which have carbon brushes and a commutator, wherein the carbon brushes on the commutator effect commutation, i.e. a change of an energized current branch. Such universal electric motors are DC motors, for example. The carbon brushes are arranged in a carbon cartridge, also referred to as sliding-contact holder, and are positioned on the commutator in the radial direction, on which commutator the electrical contacts to the current branches are arranged. In order to ensure safe electrical contact, said carbon brushes are pressed against the commutator with the aid of a brush spring acting in the radial direction.
In the case of such electric motors, the commutator and the carbon brushes are subject to considerable wear, with the result that they need to be replaced from time to time. A reduction in this service complexity is of great benefit to customers.
A brushholder comprising a carbon brush, a carbon cartridge and a brush spring forms, with its mass and rigidity properties, a vibratory system which is clamped in between the rotor and a housing of the electric motor. The vibrations of the carbon brushes occurring during operation of the electric motor have a substantial influence on wear and influence the contact-making and commutation response of the motor directly.
Forces exciting this vibratory system can be introduced both from the rotor and from the housing. In this case, all of the deviations of an optimal concentric true running of the rotor with respect to the carbon brushes in the radial and axial directions have the effect of exciting vibrations. The reasons for this are, for example, irregularities of the commutator, eccentricities in the rotor, imbalance, bearing play, etc.
The quality of the guidance of the carbon brush in the carbon cartridge also has a considerable influence on the vibration response. This is because the carbon brush is arranged in its carbon cartridge with a small amount of play in order to compensate for component tolerances so that it can slide in the carbon cartridge. Although this play is small, it is sufficient to cause the carbon brush to reel, i.e. to cause the carbon brush to vibrate within the accommodating area of the carbon cartridge. In the process, the carbon brush is at least partially lifted off from the commutator, as a result of which uneven abrasion occurs. In addition, this may result in sparking, i.e. in the formation of an arc between the carbon brush and the commutator, since the current density increases owing to the reduced contact area. Owing to the severe thermokinetic loading, the carbon brush and the commutator are subjected to very severe wear in the case of sparking.
Particularly high levels of excitation of vibrations of the commutation system are caused in the case of impact hammer drills and jackhammers, which naturally have high vibration amplitudes. By virtue of the introduction of anti-vibration handles, the damping influence of the operator on the machine vibrations has additionally been reduced, with the result that the vibration input into the commutation system has rather tended to increase in the case of new device generations.
The excitations lead not only to axial, radial and tangential vibration amplitudes of the carbon brushes relative to the commutator and therefore to excessive wear, but also impair the true running of the commutator, with the result that fluctuations in speed can arise and the control quality of the speed regulation is impaired. In addition, the impairment of the commutation efficiency also results in an overall impaired motor efficiency. Furthermore, an increased amount of complexity in terms of interference suppression owing to relatively poor EMC (electromagnetic compatibility) values should be expected.
The document DE 10 2009 025 340 A1 discloses an electric hand tool comprising a brushholder for accommodating a sliding brush which comprises a spiral spring. The spiral spring is fixed in place with an inner end such that a spring pressing force relative to the brushholder can be generated by rotation of the spiral spring. With the outer end of the spiral spring, said spiral spring rests on an inclined or tilted top bevel of the sliding brush in such a way that the spiral spring effects a spring pressure which has not only a radial component, but also a tangential and an axial component. As a result, tilting of the sliding brush to and fro within the accommodating area of the brushholder is reduced.